1. Field of the Invention
The present invention relates to image-forming apparatuses for forming characters and images on a recording medium, and in particular, relates to an inkjet image-forming apparatus ejecting ink for recording. The inkjet image-forming apparatus generally uses a recording head having a plurality of ink ejection nozzles to eject ink from an orifice at the end of each nozzle for recording. The present invention also relates to a technique for complementarily recording a region where recording is difficult due to a non-ejection nozzle among the plurality of ink ejection nozzles using a normal nozzle.
2. Description of the Related Art
Recently, inkjet recording apparatuses have been rapidly developed and widely used for recording apparatuses of PCs, copying machines, and word processors. In particular, the recording apparatus has been colorized along with the advancement in capabilities of office automation equipment, so that various color inkjet recording apparatuses have been developed. Also, inkjet recording apparatuses for fine digital pictures taken by digital cameras have become widely used.
An inkjet recording apparatus generally includes a carriage having a recording head and an ink tank, a conveying unit for conveying a recording sheet, and a controlling unit for controlling these units to record images on a recording medium by ejecting ink droplets from a plurality of orifices of the recording head. In a generally known serial recording apparatus, a recording sheet is scanned with the recording head in a direction (referred to as a main scanning direction below) perpendicular to a conveying direction of the recording sheet (referred to as a counter-scanning direction below) for recording. In this serial recording apparatus, by repeating the scanning with the recording head in the main scanning direction and intermittent conveying of the recording sheet during non-recording in the counter-scanning direction, the recording on the entire recording medium is completed. The color inkjet recording apparatus has a recording head capable of ejecting multicolor ink. By overlapping ink droplets ejected to correspond to a plurality of colors from the recording head, color images are formed.
In the inkjet recording apparatus, a heater element and a piezoelectric element are widely known as ink ejecting means. In the method using the heater element, there is provided the heater element (electrothermal conversion member) arranged in the vicinity of an orifice, and by applying an electric signal to the heater element, ink is locally heated so as to generate bubbles so that ink is ejected by a pressure produced when the bubbles are generated. In the method using an electromechanical conversion member such as the piezoelectric element, a pressure is mechanically produced in ink using an electro-pressure converter such as the piezoelectric element to eject the ink.
Since in the inkjet recording apparatus, a recording medium is scanned with the recording head with the recording head out of contact with the recording medium, noise is reduced in comparison with known recording systems such as a thermal recording system and a thermal transfer recording system. As the inkjet system is recording by bonding ejected ink on a recording medium, this system has an advantage that images can be recorded on various recording media, such as coated paper and normal paper. Due to such advantages, the inkjet recording apparatus has become widely used. Another advantage of an inkjet system is that recording is easily achieved on a large-sized recording medium, and thus, can be used to record on such large items such as an advertisement poster or fabric cloth, such as clothing. As described above, the inkjet recording apparatus has been recognized by various industrial fields as excellent recording means. However, more highly improved-quality images are being required, as well as faster printing times.
As for a recording method of the color inkjet recording apparatus, a method using three-color inks of cyan (C), magenta (M), and yellow (Y) has been known. Also, four-color inks have been generally known, in which in addition to the three-color inks, black (Bk) ink is added.
A monochrome inkjet recording apparatus for recording by ejecting only the black ink is mainly used for recording characters and line drawing. On the other hand, in the color inkjet recording apparatus for recoding color images, various properties, such as color-forming properties, gradation, and uniformity, are demanded as performances of the apparatus.
The quality of recorded images largely depends on the performance of the recording head. The shape of the recording head orifice and the ejecting means for ejecting ink may exhibit variations during the manufacturing process of the recording head. The variations during the manufacturing process may have a bad influence upon the amount of ejected ink and the ejection direction, resulting in unevenness in density of finally recorded images due to the slight variation of each orifice.
The variations in ejection amount and direction for each orifice periodically generate a “white portion” where 100% area factor is not satisfied, and a high-density portion caused by excessively overlapped dots in recorded images so as to exhibit density unevenness. Also, excessive variations in the ejection direction may generate white streak in recorded images. These phenomena are normally perceived as the density unevenness by human eyes.
A multi-path recording system has been proposed for solving such a density unevenness problem. This multi-path recording system is described with reference to FIGS. 7A to 7C.
FIGS. 7A to 7C are drawings for illustrating the known multi-path recording system. For discussion purposes, in the drawings, a single-color recording head (an inkjet head 13) with eight nozzles (orifices) is described.
FIG. 7A illustrates a first scanning recording scanned with the recording head in the main scanning direction. In the drawing, arrow Y denotes the counter-scanning direction and arrow X denotes the main scanning direction.
In the first scanning with the recording head in the main scanning direction, a position of the recording head along the counter-scanning direction is shown by numeral 501. In the first scanning, using four nozzles at the forefront of the eight nozzles, a staggered pattern (pattern indicated by positions of symbol ●) is recorded. In the description herein, a recording medium is conveyed in the upper direction of the drawing so that the lowest nozzle of the recording head 13 is the forefront of the nozzle line.
Then, the recording medium is conveyed in the counter-scanning direction by half of the recording width of the recording head 13. Since in the example of FIGS. 7A to 7C, the recording width of the recording head 13 corresponds to eight dots, the recording medium is conveyed by the width of four dots.
Consequently, a second scanning is performed with the recording head 13. FIG. 7B is an explanatory view illustrating the second scanning. As shown in the drawing, in the second scanning, a staggered pattern reverse to that of the first scanning (pattern indicated by positions of symbol ◯) is recorded using all eight nozzles of the recording head. By the recording in the second scanning, the recording a recording region corresponding to half of the recording width of the recording head 13 is completed. In the recording of the second scanning shown in FIG. 7B, the conveying operation after the first scanning is controlled so that the recording head 13 is displaced in the counter-scanning direction (arrow Y direction) relative to the position of the recording head 13 in the counter-scanning direction in the first scanning. Hence, in the second scanning, the recording head 13 is located at a position indicated by numeral 502 while in the first scanning, the recording head 13 is located at the position 501 relative to the recording medium, so that the recording medium is scanned in arrow X direction of FIG. 7A (main scanning direction) so as to record images.
Similarly, after the second scanning, the medium is conveyed in the counter-scanning direction by a distance corresponding to half of the recording width of the recording head 13. Consequently, a third scanning is performed with the recording head 13 for recording. The third scanning is described with reference to FIG. 7C. During the third scanning, the position of the recording head 13 is indicated by numeral 503, and is displaced in the counter-scanning direction by a distance corresponding to the recording width of the recording head 13 relative to the first scanning, while being displaced in the counter-scanning direction by a distance corresponding to half of the recording width of the recording head 13 relative to the second scanning. Also, in the third scanning, in the same way as the preceding scanning recordings, the medium is scanned with the recording head 13 in the main scanning direction so as to record images. In the third scanning, in the same way as in the first scanning, dots are recorded in the same staggered pattern (pattern indicated by positions of symbol ●). The execution of the first to the third scanning brings to completion of the recording images of the region width corresponding to numeral 505 in the drawing.
As described above, in the example of the multi-path recording system shown in FIGS. 7A to 7C, while the medium being conveyed in a four-dot unit, the staggered pattern and the reverse staggered pattern are alternately recorded in each scanning. By this recording operation, the four-dot width unit recording region is completed for each scanning. In such a manner, one line (recording region where is once scanned with the recording width of the recording head) is recorded with two different nozzles, so that high-quality images with suppressed density unevenness can be formed. Since in the multi-path recording system, the amount recorded with one time scanning can be reduced, the system has an effect suppressing bleeding (blur). Since the number of dots recorded with one time scanning can also be reduced, the system simultaneously has an effect suppressing the temperature rising of the recording head. The temperature rising of the recording head may cause ejection failure, so that the ejection failure may also be suppressed.
In the multi-path recording system mentioned above, several techniques have been known in producing data (path data) for recording (ejecting ink) corresponding to each scanning. For example, a technique for producing data by thinning out recording data using staggered/reverse staggered patterns mentioned above (fixed thinning system), a technique for producing path data by thinning out recording data using a random mask pattern having recording dots and non-recording dots arranged at random (random thinning system), and further a technique for producing path data by thinning out recording dots (data thinning system) are known. The technique for producing path data with the random mask is disclosed in Japanese Patent Laid-Open No. 05-155040.
In the recording head of the inkjet recording apparatus, if a non ink-ejecting state is left for a long time, ink is thickened in an ink flow path, especially in the vicinity of the orifice, so that the ink may not be normally ejected.
If the recording operation continues with high recording-dot ratio (high recording duty), micro bubbles are produced in ink within the ink flow path along with ink ejection. If the generated micro bubbles grow so as to remain in the ink flow path, the ejection is affected so that normal ejection may not be performed from the recording head. These bubbles may be mixed in ink in a connection part of an ink supply path in addition to those generated along with the ink ejection mentioned above. There may also be a case where ink cannot be ejected from a nozzle corresponding to a damaged or depleted ink-ejecting element and a case where ink cannot be ejected due to an unrecoverably clogged nozzle.
Such ink non-ejection reduces the reliability of the recording apparatus because a portion unable to be recorded is produced in images to be recorded. A recording technique has been known for complementing an image defect produced by such non-ejection with other normal nozzles.
For example, a recording technique for complementing the non-ejection is disclosed in Japanese Patent Laid-Open No. 2000-094662. In this publication, a method is disclosed in that using the above-mentioned multi-path recording system, a position corresponding to a non-ejection nozzle is recorded with another nozzle capable of recording with scanning according to data for the non-ejection nozzle. A method is also disclosed in that when a non-ejection nozzle is produced, a mask pattern for multi-path recording is changed or a new mask pattern is formed.
In the recording technique for complementing the non-ejection nozzle of the multi-path recording system, mask pattern data are rewritten corresponding to positions of non-ejection nozzles, so that forming new mask pattern data corresponding to the entire nozzles requires a long time.
For reducing the rewriting procedure of the mask pattern, the mask pattern may be changed so that the data of the non-ejection nozzle is allocated to only one specific nozzle. However, this method reduces the advantage reducing the unevenness obtained by the multi-path recording system. The nozzle for the complement may be used very often, resulting in deterioration of image quality.
Also, the mask pattern data may be changed so that data corresponding to non-ejection nozzles are allocated to a plurality of other nozzles for recording positions corresponding to the non-ejection nozzles. However, with increasing number of scannings until image completion in the multipath recording, more time is required for the changing procedure.
There is also a technique in that mask data are prepared for completing images with the number of paths smaller than the number of scannings (number of paths) until image completion, and only mask patterns corresponding to a plurality of other nozzles for recording the same positions as those of the non-ejection nozzles are rewritten in the prepared mask data. However, in this technique, the mask data for complementing non-ejection must be prepared to have the number of paths less than the maximum number of paths by one path. Thus, in view of probable non-ejecting nozzles, the mask data must be prepared in advance, so that storing means for storing the mask data such as a memory has to be prepared.
When the number of non-ejection nozzles is large, the readout for copying of mask data for complementing non-ejection and the writing of the mask data for practical use in the mask processing are necessary, so that this procedure needs a long time.